Store http.Request reference in endpoints.Context (see #2)

A possible solution for
https://github.com/crhym3/go-endpoints/issues/2#issuecomment-18984862
Use tokeninfo API for Bearer token validation in dev_appserver

A fix for #6
https://github.com/crhym3/go-endpoints/issues/6
Store http.Request reference in tokeninfoContext
Resolving conflicts with upstream.

I shouldn've had rebased t6-tokeninfo onto t2-httprequest, forgot I already
pushed it to remote. Sorry about that.